Substrates with patterns of metallic material have a wide variety of commercial applications. In some instances, it is desired that a conductive grid be sufficiently fine to be invisible to the unaided eye and supported on a transparent polymeric substrate. Transparent conductive sheets have a variety of uses including, for example, resistively heated windows, electromagnetic interference (EMI) shielding layers, static dissipating components, antennas, touch screens for computer displays, and surface electrodes for electrochromic windows, photovoltaic devices, electroluminescent devices, and liquid crystal displays.
The use of essentially transparent electrically conductive grids for such applications as EMI shielding is known. The grid can be formed from a network or screen of metal wires that are sandwiched or laminated between transparent sheets or embedded in substrates (U.S. Pat. Nos. 3,952,152; 4,179,797; 4,321,296; 4,381,421; and 4,412,255). One disadvantage of using wire screens is the difficulty in handling very fine wires or in making and handling very fine wire screens. For example, a 20 micrometer diameter copper wire has a tensile strength of only 1 ounce (28 grams force) and is therefore easily damaged. Wire screens fabricated with wires of 20 micrometer diameter are available but are expensive due to the difficulty in handling very fine wire.
Rather than embed a preexisting wire screen into a substrate, a conductive pattern can be fabricated in-situ by first forming a pattern of grooves or channels in a substrate and then filling the grooves or channels with a conductive material. This method has been used for making conductive circuit lines and patterns by a variety of means, although usually for lines and patterns on a relatively coarse scale. The grooves can be formed in the substrate by molding, embossing, or by lithographic techniques. The grooves can then be filled with conductive inks or epoxies (U.S. Pat. No. 5,462,624), with evaporated, sputtered, or plated metal (U.S. Pat. Nos. 3,891,514; 4,510,347; and 5,595,943), with molten metal (U.S. Pat. No. 4,748,130), or with metal powder (U.S. Pat. Nos. 2,963,748; 3,075,280; 3,800,020; 4,614,837; 5,061,438; and 5,094,811). Conductive grids on polymer films have been made by printing conductive pastes (U.S. Pat. No. 5,399,879) or by photolithography and etching (U.S. Pat. No. 6,433,481). These prior art methods have limitations. For example, one problem with conductive inks or epoxies is that the electrical conductivity is dependent on the formation of contacts between adjacent conductive particles, and the overall conductivity is usually much less than that of solid metal. Vapor deposition of metal or electroplating is generally slow and often requires a subsequent step to remove excess metal that is deposited between the grooves. Molten metal can be placed in the grooves but usually requires the deposition of some material in the grooves that the metal will wet. Otherwise the molten metal will not penetrate nor stay in the grooves due to surface tension of the molten metal.
In addition to conductive grids, substrates supporting patterns of conductive materials in the form of electrical circuits are also useful. Flexible circuitry is used in the support and interconnection of electronic components, as well as in the fabrication of sensors. Examples of sensors include environmental sensors, medical sensors, chemical sensors, and biometric sensors. Some sensors are preferably transparent. As in the case of conductive grids, flexible circuits on polymer film substrates are often fabricated using photolithography, which includes multiple steps of photoresist placement, exposure, development, and removal. Alternative methods that do not require such expensive equipment and so many fabrication process steps are desired in the industry.
Circuits have been made by placing metal powder into grooves followed by compacting the powder to enhance electrical contact between the particles. Lillie et al. (U.S. Pat. No. 5,061,438) and Kane et al. (U.S. Pat. No. 5,094,811) have used this method to form printed circuit boards. However, these methods are not practical for making fine circuits and fine metal patterns. On a fine scale, replacing or re-registering the tool over the embossed pattern to perform the metal compaction can be difficult. For example, a sheet with a pattern of 20 micrometer wide channels would require that the tool be placed over the pattern to an accuracy of roughly 3 micrometers from one side of the sheet to the other. For many applications, the sheet may be on the order of 30 cm by 30 cm. Dimensional changes due to thermal contraction of a thermoplastic sheet are typically about 1 percent or more during cooling from the forming temperature to room temperature. Thus, for a 30 cm by 30 cm sheet, a contraction of 1 percent would result in an overall shrinkage of 0.3 cm. This value is 1000 times larger than the 3 micrometer placement accuracy needed, making accurate repositioning of the tool difficult.